The 37-year-old Danish citizen is accused of killing four women and a man on Wednesday night in the southern town of Kongsberg.
The suspect in the Norway bow and arrow rampage which left five people dead had been flagged for possible signs of radicalisation, police have said.
Officers had been worried about signs the man, who is a Muslim convert, may have been radicalised, police chief Ole Bredrup Saeverud said at a news conference.
According to prosecutor Ann Iren Svane Matthiassen, he admitted carrying out the attack in Kongsberg, to the southwest of capital Oslo which also left two others wounded.
Thomas Nilsen lives near the scene of the attacks and said he came out of his house after hearing what he initially thought was an argument.
"I thought it was war, because it was so intense." he told NRK.
"But then I heard a scream I have never heard before. It was a scream that burned into the soul. I will never forget that. I perceived it as a death cry."
After walking outside his home he saw people had begun to gather to start administering life-saving first aid.
"In the end, there were 20 to 30 police cars, ambulances and fire trucks there," he told VG newspaper.
"Then I did not stay around to look."
